1. Field of the Invention
The present invention relates to a semiconductor die and, more particularly, to a semiconductor die with reduced RF attenuation.
2. Description of the Related Art
A semiconductor die is a well-known structure that includes a substrate material, such as single-crystal silicon, a metal interconnect structure that sits on top of the substrate material, and a number of electronic devices that are formed in and on the substrate material and/or in the metal interconnect structure. The metal interconnect structure electrically connects the electronic devices together to realize an electronic circuit.
FIG. 1 shows a perspective view that illustrates an example of a prior-art semiconductor die 100. As shown in FIG. 1, semiconductor die 100 includes a substrate material 110, and a metal interconnect structure 112 that sits on substrate material 110. Metal interconnect structure 112, in turn, includes a non-conductive material 114, and a number of layers of metal, including a top layer of metal, that are isolated by non-conductive material 114.
In the FIG. 1 example, only the top layer of metal is shown. In this example, the top layer of metal has a number of metal bond pads 120, including metal bond pad 120-1 and metal bond pad 120-2, and a number of metal traces 122, including metal trace 120-1 and metal trace 120-2, that extend away from the metal bond pads 120. Each metal bond pad 120 provides a point for an external electrical connection, while each metal trace 122 provides a signal path. Although not shown, the metal traces 122 are electrically connected to the electronic devices that are formed in and on substrate 110 material and/or in the metal interconnect structure 112.
In operation, when an RF signal is applied to metal bond pad 120-1, the RF signal propagates down metal trace 122-1. The RF signal on metal bond pad 120-1 and metal trace 122-1, in turn, is undesirably capacitively coupled to substrate material 110. In other words, as shown in FIG. 1, metal bond pad 120-1 and metal trace 122-1 function as the top plate of a parasitic capacitor 130, substrate material 110 functions as the bottom plate of parasitic capacitor 130, and non-conductive region 114 functions as the dielectric layer of parasitic capacitor 130.
Substrate material 110, in turn, is electrically conductive. As a result, as shown in FIG. 1, the RF signal capacitively coupled to substrate material 110 is also resistively coupled to a substrate bias node 132, such as ground, by a resistance 134. Thus, since a capacitor functions as a short circuit to a time varying signal, the RF signal is effectively connected to ground by way of resistance 134. As a result, a parasitic signal path exists from metal bond pad 120-1 and metal trace 120-1 to ground by way of capacitor 130 and resistance 134 that can significantly attenuate the RF signal propagating down metal trace 120-1.
In addition, the RF signal capacitively coupled to substrate material 110 is also resistively coupled to a region 136 of substrate 110 by a resistance 138. In region 136, the RF signal can be capacitively coupled to metal bond pad 120-2 and metal trace 122-2 by way of a parasitic capacitor 140.
As a result, a second parasitic signal path exists from metal bond pad 120-1 and metal trace 122-1 to metal bond pad 120-2 and metal trace 122-2 by way of capacitor 130, resistance 138, and capacitor 140 that can significantly attentuate the RF signal. Further, an RF signal capacitively coupled to metal bond pad 120-2 and metal trace 122-2 by way of parasitic capacitor 140 degrades and interferes with an RF signal that is placed on metal bond pad 120-2 and metal trace 122-2. One approach to reducing the attenuation associated with the parasitic signal paths is to increase the thickness (height) of non-conductive material 114 so that metal bond pad 120-1 and metal trace 122-1 lie further away from the top surface of substrate material 110. Another approach is to reduce the coupling area by reducing the widths of the metal traces. A further approach is to utilize a high-resistance substrate material that has the effect of substantially increasing the values of resistance 134 and resistance 138.
Although each of these approaches provides some reduction in the attenuation of an RF signal, there is a need for an additional approach to reducing the attenuation of an RF signal that propagates down a metal trace of a metal interconnect structure after being applied to a metal bond pad.